An active-matrix screen should be understood to be a screen in which a circuit with transistors and storage capacitor(s) is associated with each pixel of the matrix, enabling a display control circuit, also transistor-based, to individually drive each pixel. This display control circuit which in reality comprises a plurality of circuits for addressing the rows, columns and common electrode of the matrix, is a circuit that is generally integrated on the same substrate at the periphery of the active-matrix zone.
The transistors employed in these screens are field-effect transistors, in so-called thin-film technology, based on amorphous silicon. The conduction characteristics of these transistors can change significantly according to the working operating conditions.
In particular, the mobility of the charge carriers in the amorphous silicon varies with the temperature: with current technologies, it thus changes from 0.1 cm2/V/s at −40° C. to 0.75 cm2/V/s at 70° C. Also, the leakage current of the transistors tends to increase with the light received by these transistors. Such is notably the case in the liquid crystal screens, according to the level of the lighting supplied by the liquid crystal backlighting source: this intensity indeed varies according to the ambient brightness conditions (day or night ambience).
For some applications, notably in the transport field (avionics, motor vehicles, maritime), the screens need to be able to work in highly variable conditions, without notable degradation of the display quality. In particular, they have to be operational over a wide temperature range, which can extend from minus 40 to plus 70 degrees Celsius for example for applications in the avionics field.
These variable and severe operating conditions are reflected in variations of the conduction parameters of the transistors. For example, after a long period of operation at high temperature, a few hundreds of hours, the threshold voltage of the transistors is temporarily increased. If it is assumed that the temperature then drops, the mobility of the carriers drops also, but the threshold voltage of the transistors at that moment is still high because of the previous high-temperature episode.
Also, to be able to control these transistors reliably, in the on state and in the off state, regardless of the immediate conduction conditions of the transistors, transistors are used which are defined with a geometry (ratio of the width to the length of the transistor channel) greater than that normally necessary. The transistors are said to be overdimensioned.
This overdimensioning of the transistors necessitates the use of equally greater values for the associated coupling and compensation capacitors and of higher power supply voltages for controlling these elements. Thus, in the avionics field, the power supply voltage is of the order of +33 volts and the maximum voltage amplitude for controlling the pixel capacitor is of the same order.
This overdimensioning of the components presents a number of drawbacks.
With respect to the aspects affecting manufacture, the overdimensioning is reflected in an increased surface area; hence a greater bulk of the driver circuits at the periphery of the panel; also, there is a greater risk of manufacturing defect commensurate with this increased surface area.
With respect to working operation, there is a true difficulty in stabilizing the output state of these driver circuits throughout the temperature range. In practice, these outputs oscillate when temperature rises. This is explained by the greater leakage currents; high current demands necessary to charge the higher capacitors, in sufficiently short times; a rapid drift of the threshold voltages because of the high voltage applied.
These oscillations can lead to perceptible flickers on the image displayed, which damage the “cosmetic” quality of the display.
It is known practice to reduce these oscillations by deliberately degrading, in manufacture, the threshold voltages of the transistors. However, how to do so in a perfectly controlled manner is not known. Furthermore, these degradation techniques reduce the life of the transistors, therefore of the screens.
Finally, these screens consume more because of the power supply and control voltage levels used.